The present invention relates to a hot-dip galvanized steel sheet used for automotive structural members, mechanical structural parts, and the like, and a method for producing the same.
In order to improve fuel economy and safety on collision, a high-tensile strength steel sheet has been demanded for vehicle body structural members and suspension members, and a high strength has been required since a long time ago. In addition, in recent years, a hot rolled steel sheet used for vehicle body structural members and suspension members is required to have excellent press formability, especially high ductility, because it is subjected to severe forming consisting mainly of bulging. In this situation, dual-phase structure type hot rolled steel sheets, basically having a microstructure consisting of ferrite and martensite, have been developed.
Furthermore, a steel sheet obtained by hot-dip galvanizing the dual-phase structure type hot rolled steel sheet having both high ductility and corrosion resistance has been demanded, and has been disclosed in Unexamined Japanese Patent Publication No. 56-142821. The steel sheet disclosed in this Publication is characterized in that a steel sheet containing 0.15% or less of C and 1.0 to 2.5% of Mn+Cr by weight % as basic components and the balance of Fe and unavoidable impurities is caused to have a dual-phase structure by a continuous hot-dip galvanizing line (hereinafter, referred to as CGL) on which a pre-plating heating temperature, cooling rate before plating bath, alloying temperature, and cooling rate after alloying are specified in detail.
Specifically, after dual-phases of ferrite phase and austenite phase are formed in the process of pre-plating heating, the austenite phase is changed to a martensite phase by hardening on the CGL.
However, in order to secure hardenability on the CGL line, an alloy element must be added as a steel component, or the line speed of CGL must be increased. The addition of an alloy element increases the cost of steel. Also, for many CGLs, hardenability cannot be secured at a line speed determined from the security of stability of zinc deposition control and the restriction of reaction rate for alloying.
On the other hand, a high-strength hot-dip galvanized steel sheet having a tensile strength exceeding 440 MPa, which has advantages of excellent rust preventing property and high proof stress, has been used widely for construction members, mechanical structural parts, automotive structural parts, and the like. Therefore, a great number of inventions relating to the high-strength hot-dip galvanized steel sheet have been disclosed. In particular, since a need for workability has increased as the application range extends, many inventions relating to a high-strength hot-dip galvanized steel sheet having high workability have been disclosed, for example, in Unexamined Japanese Patent Publication Nos. 5-311244 and 7-54051.
In recent years, while a need for workability of a steel sheet as is manufactured has increased, attention has been paid to the properties of weld portion as a need for a product. This is because as the technology to which the steel sheet is applied expands, a steel sheet is fabricated in a state of including a weld portion as in the case of tailored blank material, or a requirement for high-speed deformation behavior of a structural member including a weld portion becomes stringent.
However, the above-described conventional high-strength a hot-dip galvanized steel sheet has a serious drawback in that a weld heat-affected zone (hereinafter, referred to as HAZ) softens at the time of welding because the main strengthening mechanism generally uses a low-temperature transformation phase such as martensite and bainite obtained by quenching of austenite phase. Such softening phenomenon occurring at the time of welding leads to decreased formability for, for example, a tailored blank material, and also causes a decrease in properties for structural member such as deformation strength, rupture strength, and high-speed deformation strength even when the steel sheet is used for other applications.
It is an object of the present invention to provide a method for manufacturing a hot-dip galvanized steel sheet with high workability without the use of an expensive alloy element and without being subject to any restriction of CGL facility, and a steel sheet manufactured by the manufacturing method.
To achieve the object, the present invention provides a hot-dip galvanized steel sheet comprising:
a steel sheet containing 0.04 to 0.12% of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less of S, 0.05 to 1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol. Al, and 0.01% or less of N by weight %;
the steel sheet having a structure consisting essentially of ferrite and martensite; and
a hot-dip galvanizing layer formed on the steel sheet.
The steel sheet may be a hot rolled steel sheet or a cold rolled steel sheet.
Further, the present invention provides a method for producing for a hot-dip galvanized steel sheet, comprising the steps of:
rough rolling a steel containing 0.04 to 0.12% of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less of S, 0.05 to 1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol. Al, and 0.01% or less of N by weight %;
finish rolling the rough rolled steel at a temperature not lower than the Ar3 point;
coiling the finish rolled steel at a temperature of 700xc2x0 C. or less; and
hot-dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3.
It is another object of the present invention to provide a new high-strength hot-dip galvanized steel plate having a property such that a change in hardness of HAZ is very small in welding such as laser welding, mush-seam welding, or arc welding, and a method for producing the same.
To achieve the object, the present invention provides a hot-dip galvanized steel sheet comprising:
a steel sheet containing 0.04 to 0.13% of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05% or less of sol. Al, 0.007% or less of N, 0.05 to 0.5% of Mo, and 0.2% or less of Cr by weight %;
the steel sheet having a structure consisting essentially of ferrite having an average grain size of 20 xcexcm or less and martensite with a volume percentage of 5 to 40%; and
a hot-dip galvanizing layer formed on the steel sheet.
The steel sheet may be a hot rolled steel sheet or a cold rolled steel sheet.
Further, the present invention provides a method for producing a hot-dip galvanized steel sheet, comprising the steps of:
rolling a steel containing 0.04 to 0.13% of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05% or less of sol. Al, 0.007% or less of N, 0.05 to 0.5% of Mo, and 0.2% or less of Cr by weight % to manufacture a strip;
pickling the strip; and
performing a continuous hot-dip galvanizing, said hot-dip galvanizing comprising the steps of:
soaking the pickled strip at a temperature of 750 to 850xc2x0 C.;
cooling the soaked strip to a temperature range of 600xc2x0 C. or less at a cooling rate of 1 to 50xc2x0 C. per second;
hot-dip galvanizing the cooled strip; and
cooling the galvanized strip so that the residence time at 400 to 600xc2x0 C. is within 200 seconds.